Immersion refractometer

Abstract

A device for measuring the absolute value of the refractive index of a liquid by immersion uses the optical properties of a cylindrical waveguide with a solid core and normal angle of incidence of the light source. The device consists of a transparent tube forming the enclosure of the waveguide, impervious to the surrounding liquid and partially filled with a transparent solid or liquid material of appropriate index of refraction, a fiber optic means of inputting light via a LED or laser and fiber optic means of coupling emerging light to a photodetector. The emerging light intensity is a function of the index of refraction of the surrounding liquid.

We claim: 
     
       1. A device for quantitatively measuring the refractive index of a liquid comprising:
 (a) a light source configured to produce light with an illumination wavelength; 
 (b) an illumination fiber bundle comprising a plurality of optical fibers, said illumination fiber bundle having first and second ends, and mounted with the light source such that light from the light source is communicated to the first end of the illumination fiber bundle; 
 (c) a transparent tube having first and second ends, comprising a material that is impervious to the liquid and that has a refractive index that is much greater than the refractive index of the liquid, mounted with the fiber bundle such that light is communicated between the second end of the illumination fiber bundle and the first end of the transparent tube; 
 (d) a core material disposed in the core of the transparent tube comprising a material that is transparent to light at the illumination wavelength and has a refractive index approximately equal to or greater than the refractive index of the liquid, disposed within the measuring tube near the second end of the measuring tube; 
 (e) a mirror mounted at the second end of the transparent tube such that light propagating through the transparent tube to the second end thereof is reflected by the mirror into the transparent tube toward the first end thereof; 
 (f) a detection fiber bundle , having first and second ends, comprising a plurality of optical fibers, mounted with the measuring tube such that light is communicated between the first end of the transparent tube and the first end of the detection fiber bundle; 
 (g) a light detector configured to produce a signal responsive to the intensity of light at the illumination wavelength, mounted with the detection fiber bundle such that light is communicated from the second end of the detection fiber bundle to the light detector; 
 (h) an analysis system configured to determine the refractive index of the liquid responsive to the signal. 
 
     
     
       2. The device of  claim 1  wherein the refractive index of the core material is equal to or exceeds the upper limit of the refractive index range of the liquid. 
     
     
       3. The device  claim 1  wherein the illumination fiber bundle and the detection fiber bundle are bundled together within a ferrule such that light enters and exits at the first end of the transparent tube. 
     
     
       4. The device of  claim 1  further comprising a reflecting mirror at the end of the transparent tube positioned orthogonally to the optical axis of the transparent tube such that light traverses the core material twice. 
     
     
       5. The device of  claim 1  wherein the illumination fiber bundle is oriented with its long axis parallel to the long axis of the transparent tube axis without regard to specific angle of impingement on the interface between the transparent tube and the core material. 
     
     
       6. The device of  claim 1  further comprising an enclosure, wherein elements (a) through (g) of the device mount with the enclosure, and wherein the enclosure is a cap for an open port battery. 
     
     
       7. The device of  claim 1  further comprising a temperature sensing element mounted with device such that the temperature sensing element is responsive to a liquid in contact with the portion of the transparent tube containing the measurement material. 
     
     
       8. The device of  claim 1  wherein the illumination fiber bundle further comprises a ferrule enclosing the illumination fibers and made of a light absorbing material. 
     
     
       9. The device of  claim 1  wherein the light source is configured to produce infrared light with wavelengths >700 nm. 
     
     
       10. The device of  claim 1 , wherein the light detector signal comprises an electrical signal, and wherein the device further comprises a temperature sensor configured to determine the temperature of the liquid, and wherein the analysis system comprises:
 (j) signal conditioning electronics configured to amplify the electrical signal; 
 (k) a model relating intensity of light detected and liquid temperature to refractive index of a liquid surrounding the potion of the measuring tube containing the measurement material. 
 
     
     
       11. A method of determining the refractive index of a liquid, comprising:
 providing a device as in  claim 1 ; 
 placing the transparent tube in the liquid such that the portion of the transparent tube containing the core material is surrounded by the liquid; 
 using the light source to produce light that travels through the illumination fiber bundle, the transparent tube, the detection fiber bundle, and then to the light detector; 
 determining the refractive index of the liquid from the signal from the light detector. 
 
     
     
       12. A method as in  claim 11 , further comprising using the device to determine the refractive index of a material with a known refractive index surrounding the core material when the device is not in contact with the liquid, and using the refractive index determined to adjust the refractive index determined for the liquid. 
     
     
       13. The device of  claim 1 , wherein the light source further comprises a focusing lens configured to communicate the light from the light source to the illumination bundle. 
     
     
       14. The device of  claim 1 , wherein the liquid comprises acid, and wherein the transparent tube is impervious to acid.